diff --git a/latex/content.tex b/latex/content.tex index 79e1e28..5699d70 100644 --- a/latex/content.tex +++ b/latex/content.tex @@ -312,25 +312,25 @@ new constraints on the first bin can be expressed as follows : \subsection{La giga demo} -Let $ k \in \mathbb{N} $. Let $ (U_n)_{n \in \mathbb{N}} $ be a sequence of +Let $ k \geq 2 $. Let $ (U_n)_{n \in \mathbb{N}^*} $ be a sequence of independent random variables with uniform distribution on $ [0, 1] $, representing the size of the $ n $-th item. Let $ i \in \mathbb{N} $. $ T_i $ denotes the number of items in the $ i $-th bin. We have that -\begin{equation} +\begin{equation*} T_i = k \iff U_1 + U_2 + \ldots + U_{k-1} < 1 \text{ and } U_1 + U_2 + \ldots + U_{k} \geq 1 -\end{equation} +\end{equation*} Let $ A_k = \{ U_1 + U_2 + \ldots + U_{k} < 1 \}$. Hence, -\begin{align*} - % TODO = k +\begin{align} + \label{eq:prob} P(T_i = k) & = P(A_{k-1} \cap A_k^c) \\ & = P(A_{k-1}) - P(A_k) \qquad \text{ (as $ A_k \subset A_{k-1} $)} \\ -\end{align*} +\end{align} We will try to show that $ \forall k \geq 1 $, $ P(A_k) = \frac{1}{k!} $. To do so, we will use induction to prove the following proposition \eqref{eq:induction}, @@ -373,20 +373,29 @@ $ f_{U_{k-1}}(y) = 1 $. We can then integrate by parts : \begin{align*} P(S_k < a) - & = \int_{0}^{a} f_{S_{k-1}}(x) \cdot \left( \int_{0}^{a-x} 1 dy \right) dx \\ - & = \int_{0}^{a} f_{S_{k-1}}(x) \cdot (a - x) dx \\ - & = a \int_{0}^{a} f_{S_{k-1}}(x) dx - \int_{0}^{a} x f_{S_{k-1}}(x) dx \\ + & = \int_{0}^{a} f_{S_{k-1}}(x) \cdot \left( \int_{0}^{a-x} 1 dy \right) dx \\ + & = \int_{0}^{a} f_{S_{k-1}}(x) \cdot (a - x) dx \\ + & = a \int_{0}^{a} f_{S_{k-1}}(x) dx - \int_{0}^{a} x f_{S_{k-1}}(x) dx \\ & = a \int_0^a F'_{S_{k-1}}(x) dx - \left[ x F_{S_{k-1}}(x) \right]_0^a - + \int_{0}^{a} x F_{S_{k-1}}(x) dx \qquad \text{(IPP)} \\ + + \int_{0}^{a} F_{S_{k-1}}(x) dx \qquad \text{(IPP : }x, F_{S_{k-1}} \in C^1([0,1]) \\ & = a \left[ F_{S_{k-1}}(x) \right]_0^a - \left[ x F_{S_{k-1}}(x) \right]_0^a - + \int_{0}^{a} \frac{x^{k-1}}{(k-1)!} dx \\ - & = \left[ \frac{x^k}{k!} \right]_0^a \\ - & = \frac{a^k}{k!} \\ + + \int_{0}^{a} \frac{x^{k-1}}{(k-1)!} dx \\ + & = \left[ \frac{x^k}{k!} \right]_0^a \\ + & = \frac{a^k}{k!} \\ \end{align*} -We have shown that $ (\mathcal{H}_{k}) $ is true, so by induction, $ \forall k \geq 1 $, +\paragraph{Conclusion} We have shown that $ (\mathcal{H}_{k}) $ is true, so by induction, $ \forall k \geq 1 $, $ \forall a \in [0, 1] $, $ P(U_1 + U_2 + \ldots + U_{k} < a) = \frac{a^k}{k!} $. Take -$ a = 1 $ to get $ P(U_1 + U_2 + \ldots + U_{k} < 1) = \frac{1}{k!} $. +$ a = 1 $ to get + +\[ P(U_1 + U_2 + \ldots + U_{k} < 1) = \frac{1}{k!} \] + +Finally, plugging this into \eqref{eq:prob} gives us + +\[ + P(T_i = k) = P(A_{k-1}) - P(A_{k}) = \frac{1}{(k-1)!} - \frac{1}{k!} \qquad \forall k \geq 2 +\] +